<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
    <title>Document</title>
</head>
<body>
    <div id="app">
        <h1>全民:{{fullName.toLowerCase()}}</h1>
    </div>
    <script>

        Vue.config.productionTip = false

        const vm = new Vue({
            el:'#app',
            data(){
                return{
                    firstName:'MEI',
                    lastName:'QINGJUN',
                    fullName:''
                }
            },
            watch:{
                firstName:{
                    immediate: true,
                    handler(){
                        this.fullName = this.firstName + '' + this.lastName
                    }
                },
                lastName:{
                    immediate: true,
                    handler(){
                        this.fullName = this.firstName + '' + this.lastName
                    }
                }
            }
        })
    </script>
</body>
</html>